C elegans discriminates between the different stimuli of nose touch, high osmolarity, and volatile repellents. These stimuli are selectively detected and separately transduced by the ASH sensory neurons, and response is coordinated by a simple, ASH-activated neural circuit. Understanding how sensory information is transmitted to produce a response requires understanding both the activity of the circuit and the molecular machinery that drives the activity. I will visualize the activities of the ASH neurons and their synaptic targets using different fluorescent protein probes to detect membrane depolarization, general synaptic vesicle release, and specific neuropeptide release in intact animals. To understand a component of the sensory machinery, I will clone and characterize a gene likely to be involved in the response to specific stimuli, specifically, the gene corresponding to the rt40 mutation. It has previously been hypothesized that ASH-specific neuropeptides act as neurotransmitters to separately transduce information specific to a particular stimulus. To understand separate encoding of particular stimuli at the synapse, I will generate mutations in two ASH-specific neuropeptide genes, nlp-3 and n1p-15, and analyze their effect on ASH sensory discrimination and general C. elegans behaviors. This analysis will elucidate how sensory information is selectively detected and separately processed. More broadly, it will define the role of neuropeptides within the ASH neural circuit.